Presbyopia is a condition of the human eye which increases with age and which requires correction for near distance vision. An individual suffering from this condition may also require correction for far distance vision. In efforts to provide a single lens incorporating corrections for both near and far distance, there have been developed progressive lenses.
Winthrop U.S. Pat. No. 4,861,153 issued Aug. 29, 1989, discusses various progressive lens designs of the prior art. This patentee describes the defect of many such lenses as failing to provide visual continuity being, in effect, similar to trifocal lenses. Other efforts of the prior art to produce progressive lenses are said to produce results in which the aberration level is substantial or which are objectionably astigmatic.
With the foregoing in mind, Winthrop sets out to develop a progressive addition spectacle lens with the smoothest possible distribution of dioptric power and the lowest possible level of unwanted astigmatism. In so doing, Winthrop provides a family or series of lenses in which the entire progressive surface can be considered to be generated by the curve of intersection formed between a sphere of variable radius and a corresponding cylinder of variable diameter. In the resultant lens, the distance and near vision "areas" are mathematical points so that the astigmatism is distributed over the entire area of the lens and thus is minimized. These distance and near power points are connected by an umbilic line of progressive dioptric power. These points, moreover, comprise the poles of a bipolar system of surface power contours.
While Winthrop provides a progressive lens having visual continuity and a low level of unwanted astigmatism, his teaching is limited to a single family or series of progressive lenses having the characteristics discussed hereinabove.